NHC Bis-Phenolate Aluminum Chelates: Synthesis, Structure, and Use in Lactide and Trimethylene Carbonate Polymerization

A novel
family of Al­(III) complexes supported by a tridentate,
dianionic N-heterocyclic carbene bis-phenolate ligand ((OCO)2–) was prepared via various synthetic routes, and the derived compounds
were all structurally characterized. The methane elimination reaction
of the protio ligand N,N′-bis­(2-hydroxy-3,5-di-tert-butylphenyl)-4,5-dihydroimidazolium chloride (1·H3Cl) with AlMe3 quantitatively
led to the formation of the bis-phenolate imidazolinium Al zwitterion
(1·H)­Al­(Me)­(Cl) (2), whose formulation
was established by X-ray diffraction studies. The deprotonation of
species 2 with 1 equiv of lithium diisopropylamide (LDA)
proceeded with the elimination of LiCl to afford the Al-NHC methyl
derivative [(OCO)­AlMe]2 (3), which was isolated
as a dimer, as confirmed by X-ray diffraction studies. Alternatively,
compound 3 may be accessed via a salt metathesis route
involving the reaction of the NHC bis-phenolate Li salt 1·Li2, generated in situ via reaction of 1·H3Cl with 3 equiv of nBuLi (−40 °C, THF), with 1 equiv of MeAlCl2. The serendipitous hydrolysis of compound 3 allowed
the X-ray characterization of the Al-oxo dinuclear species [(OCO)­Al-O-Al-(OCO)]
(3′), in which both Al­(III) centers adopt a distorted-trigonal-monopyramidal
geometry. The reaction of the salt 1·H3Cl with Al­(OiPr)3 afforded the corresponding
bis-phenolate imidazolinium Al zwitterion (1·H)­Al­(OiPr)­(Cl) (4), which incorporates a four-coordinate
tetrahedral Al center effectively κ2O,O-chelated by the two phenolate moieties of the OCO2– ligand. Compound 4 may be readily converted to the
Al-NHC alkoxide derivative [(OCO)­AlOiPr]2 (5) upon reaction with 1 equiv of LDA. Alternatively,
the alcoholysis of the Al-NHC methyl species 3 with iPrOH also permitted access to the derived Al alkoxide 5 and proceeds via the formation of the kinetic product (1·H)­Al­(OiPr)­(Me) (6) that
may readily eliminate methane upon heating to produce species 5. The Al alkoxide species 5 was shown to efficiently
polymerize rac-lactide and trimethylene carbonate
in a highly controlled manner for the production of narrow disperse
materials. The observed catalytic performances are in the range of
the majority of those for group 13 metal based ROP catalysts developed
thus far, and all data support the noninvolvement of the NHC moiety
in these polymerization reactions.